An E.sup.2 PROM is a nonvolatile memory device which may be programmed and erased using only electrical techniques. A common type of E.sup.2 PROM operates as an insulated gate field effect transistor having a source region, a drain region, and a gate region, known as the control gate, and an extra gate region, known as a floating gate. In one type of floating gate E.sup.2 PROM the floating gate is an electrical conductor or semiconductor, typically polycrystalline silicon, which is encapsulated by a dielectric material. Because of this encapsulation the floating gate has no direct electrical connection with other regions of the device or with structures external to the device. Therefore any change in electrical conditions on the floating gate must result from charge carrier tunneling through the dielectric material.
In a typical floating gate E.sup.2 PROM the floating gate is provided as a layer of polycrystalline silicon lying between the polycrystalline silicon control gate and the silicon substrate which includes the source, drain, and channel regions. The floating gate is said to be the poly 1 layer because it is the first layer of polycrystalline silicon formed, while the control gate is the poly 2 layer.